Bacterial superantigens are toxins secreted by some strains of staphylococcal and streptococcal bacteria which stimulate a substantial proportion of T-cells resulting in fever and severe, sometimes lethal, toxic shock. It is known that superantigens bind as globular proteins to MHC class II molecules and particular V-beta chains of the T-cell antigen receptor (TCR), and in this way bypass the specificity of the TCR for the peptide/MHC complex. Recently, however, it has been clearly demonstrated that MHC-associated peptides control binding of superantigens to class II molecules and their subsequent presentation to T-cells. This application will continue studies of the mechanisms of peptide-mediated control of T-cell activation by superantigens. Altered peptides will be used in Specific Aim 1 to determine whether the length of the C-terminal region, the ability of the peptide to promote stable versus unstable forms of class II molecules, or the specific sequence of the peptide controls presentation of toxic shock syndrome toxin-1 (TSST-1) to T-cells. In Specific Aim 2, these studies will be extended to other bacterial superantigens, which may use distinct mechanisms to distinguish among MHC antigens loaded with different peptides. Also in this aim, staphyloccal enterotoxin A (SEA) mutants will be used to look individually at superantigen binding to two distinct sites on class II. Studies in Specific Aim 3 will approach the question of the biological significance of peptide discrimination by superantigens. To test the idea that superantigens use MHC-associated peptides to distinguish distinct types of antigen presenting cells (APCs), the investigator will compare the capacity of peptides derived from various types of APCs to promote superantigen presentation and affect the quality of the T-cell response. Cells expressing single peptide/MHC complexes will be used to test the idea that peptide discrimination enhances the T-cell response by limiting the density of superantigen presentation.